1. Field of the Invention
The present invention relates to an electric power steering device to be used for vehicles such as motor vehicles.
2. Description of the Related Art
In an electric power steering device, there is known such control of a return speed that, when a steering wheel is brought into a state of returning toward a neutral position, the steering wheel appropriately returns toward the neutral position even when the self-aligning torque of the steered wheels is low. In this return speed control, a basic target return speed, which is an ideal return speed, is calculated based on a vehicle speed and a steering angle, and a steering assist torque is controlled through feedback control based on a deviation between an actual return speed and the basic target return speed so that the actual return speed reaches the basic target return speed.
The basic target return speed is calculated so as to increase in the magnitude as the magnitude of the steering angle increases. At the beginning of the return speed control, the magnitude of the deviation between the actual return speed and the basic target return speed is large. Thus, the magnitude of a feedback control amount is significantly large, and hunting is often caused in which a state of an excessive actual return speed and a state of an insufficient actual return speed with respect to the basic target return speed are alternately repeated. In order to avoid unnecessary increase/decrease in the return speed due to the hunting, for example, as disclosed in Japanese Patent Application Laid-open No. Sho 62-241768, such a configuration that as the magnitude of the deviation of the return speed increases, a gain of the feedback control is decreased is known.
In the electric power steering device disclosed in Japanese Patent Application Laid-open No. Sho 62-241768, at the beginning of the return speed control, the magnitude of the deviation of the return speed is large. Thus, the gain of the feedback control is decreased, and the magnitude of the feedback control amount does not increase to a large value. Therefore, the magnitude of the actual return speed does not become sufficiently large, and thus a decrease speed of the magnitude of the steering angle is small. Thus, the change in the basic target return speed with respect to the elapsed time is slow, and a state in which the magnitude of the deviation of the return speed is large continues for a long period. Therefore, a state in which the gain of the feedback control is set to a small value also continues for a long period. As a result, the period until the steering wheel returns to the neutral position becomes significantly long. Thus, with the electric power steering device disclosed in Japanese Patent Application Laid-open No. Sho 62-241768, the unnecessary increase/decrease in the return speed due to the hunting can be avoided, but the original purpose of the return speed control, which is to appropriately return the steering wheel toward the neutral position, cannot be attained.